1. Field of the Invention
The present invention relates to a reference voltage generating circuit for generating a constant voltage independent of a variation of a voltage of a power supply thereof, a change of an ambient temperature and the like.
2. Description of the Prior Art
FIG. 1 shows an example of a conventional reference voltage generating circuit formed in a semiconductor integrated circuit. A voltage of a power source is applied between terminals T1 and T2. A reference voltage is also withdrawn between the terminals T1 and T2. The terminal T2 is a terminal on the ground side.
FIG. 2 is a basic circuit indicating a basic principle of a conventional reference voltage generating circuit. The voltage of a power source is applied between the terminals T1 and T2. A reference voltage is withdrawn between the terminals T3 and T2. The terminal T2 is a terminal on the ground side. A basic principle of the conventional reference voltage generating circuit will be described in the following with reference to FIG. 2.
A base of a transistor Q21 is connected to a base of a transistor Q22. The transistor Q21 has a collector connected to the base thereof so that the transistor Q21 has a diode function. Further, emitters of these transistors are connected to each other through a resistor R23. The transistor Q21 is operated with a relatively large current density J1 whereas the transistor Q22 is operated with a relatively small current density J2, for example, J2=1/10.multidot.J1. A difference, .DELTA.V.sub.BE, between a base-emitter voltage of the transistor Q21 and a base-emitter voltage of the transistor Q22 is generally represented by ##EQU1## where k indicates Boltzmann's constant, T indicates an absolute temperature and q indicates the charge of an electron.
.DELTA.V.sub.BE is applied to the resistor R23. If and when a current amplification factor of the transistor Q22 is sufficiently large, a current determined by the .DELTA.V.sub.BE and the resistor R23 becomes equal to a collector current I.sub.C22 of the transistor Q22. Accordingly, the equation I.sub.C22 =.DELTA.V.sub.BE /R23 is established. Thus, a drop voltage V.sub.R22 of the resistor R22 connected to the collector of the transistor Q22 becomes ##EQU2##
On the other hand, the collector current I.sub.C22 of the transistor Q22 is applied to a base of a transistor Q23 and thus an amplified current flows through the transistor Q23. A base-emitter voltage V.sub.BE of the transistor Q23 is generally represented by ##EQU3## where V.sub.g0 indicates an extrapolation voltage of an energy band gap inherent to a semiconductor material at T=0.degree. K., n is a constant which is dependent on a manufacturing condition of a transistor, I.sub.C indicates a collector current and I.sub.C0 indicates a collector current at T=0.degree. K. Further, V.sub.BE0 indicates a base-emitter voltage at T=0.degree. K. The last two terms in the equation (3) are negligible since these terms is sufficiently small to a variation of a collector current I.sub.C at an absolute temperature. Thus, the equation (3) is briefly represented in the following. ##EQU4##
One end of the resistor R22 is connected to a terminal T3 and the other is connected to the base of the transistor Q23. An emitter of the transistor Q23 is connected to the terminal T2. Accordingly, a reference voltage V.sub.ref withdrawn between the terminals T3 and T2 is evaluated from the following equation. EQU V.sub.ref =V.sub.R22 +V.sub.BE ( 5)
Substituting the equations (1), (2) and (4) for the equation (5), ##EQU5## is obtained.
In order to evaluate a temperature coefficient of the reference voltage V.sub.ref, differentiating the equation (6) with respect to an absolute temperature T, the following equation ##EQU6## is obtained. In order for the variation of the reference voltage V.sub.ref due to a temperature to be 0, the condition ##EQU7## is needed. More particularly, ##EQU8## that is, ##EQU9## is a condition necessary for the variation of the reference voltage V.sub.ref due to an temperature to be 0.
Referring to the equations (1) and (2), the first term on the right side in the equation (7) indicates a drop voltage V.sub.R22 of the resistor R22. The second term on the right side in the equation (7) indicates a base-emitter voltage of the transistor Q23. Thus, the entire right side in the equation (7) indicates a voltage between the terminals T3 and T2, that is, a reference voltage V.sub.ref. Accordingly, in order for the equation (7) to be fulfilled so that the variation of the reference voltage due to a temperature becomes 0, EQU V.sub.ref =V.sub.g0 ( 8)
must be fulfilled. More particularly, in the circuit shown in FIG. 2, the reference voltage V.sub.ref can be maintained constant with respect to a variation of a temperature by setting V.sub.ref =V.sub.g0.
As described in the foregoing, V.sub.BE has a negative temperature coefficient (refer to the equation (4)) and .DELTA.V.sub.BE has a positive temperature coefficient (refer to the equation (1)). Accordingly, if and when these two voltages are summed in such a manner that a voltage variation due to a temperature variation is cancelled, the summed voltage becomes independent of the temperature variation. This is a principle of the conventional reference voltage generating circuit as shown in FIG. 2.
In the conventional reference voltage generating circuit structured based on the principle, V.sub.ref =V.sub.g0 shown in the equation (8) must be fulfilled. Thus, only the value equal to an extrapolation voltage of an energy band gap can be selected as a reference voltage V.sub.ref. For example, a semiconductor integrated circuit using a silicon can take only approximately 1.205 volts as a reference voltage, since an extrapolation voltage V.sub.go of an energy band gap of a silicon is 1.205 volts.
Thus, the conventional reference voltage generating circuit can have merely a single reference voltage value dependent on a semiconductor material. Thus, conventionally, in order to obtain a desired reference voltage needed in a circuit design, it is necessary to provide a level shifting circuit in a latter stage of a reference voltage generating circuit. Furthermore, in case where a voltage of a power supply is smaller than an extrapolation value of an energy band gap, there exits a serious problem that the above described conventional reference voltage generating circuit cannot be directly used.